The present invention generally relates to heat transfer and storage systems which may readily be incorporated into conventional heating systems in order to provide a heat collection and distribution system, preferably utilizing a solar collector panel for collecting solar heat energy. Electronic controls to command and regulate the various components of the system may be included. The present invention is particularly directed to a solar heat collection unit which utilizes a phase change material to store collected solar heat within the unit and a system utilizing such a unit as a component part.
As used herein the term "phase change material" means any substance or combination of substances which, when changing state, have the property of remaining at substantially a constant temperature while absorbing or yielding up significant amounts of heat energy. For example, H.sub.2 O changes state, from liquid (water) to solid (ice) or from solid (ice) to liquid (water), substantially at 32 degrees Fahrenheit (0 degrees Celsius). H.sub.2 O will remain at a constant temperature substantially at 32 degrees Fahrenheit until the change of state is complete. Approximately 144 BTU will be absorbed by a pound of H.sub.2 O during its change of state from ice to water and approximately 144 BTU will be yielded up by a pound of H.sub.2 O during its change of state from water to ice.
As used herein the term "paraffin waxes" means one or more phase change materials selected from the group of straight-chained hydrocarbon materials unusually obtained by chilling paraffin distillate to remove the heavy oils. These materials may generally be represented by the chemical formula (C.sub.n H.sub.(2n+2) where n equals from about 26 to about 30.
As used herein the term "latent heat" means the heat per unit mass or mole required to produce a change of phase of a given material at a constant temperature and pressure. Often an adjective is added to specify the type of phase transistion involved and the adjective "latent" is omitted, e.g. (latent) heat of evaporation, (latent) heat of sublimation, (latent) heat of fusion, etc.
As used herein the term "sensible heat" means the heat absorbed per unit mass or mole of a given material in raising the temperature of the material without effecting a change in the phase of the material.
Systems for collecting, storing and transferring solar energy for utilization in conjunction with residential water heaters are well-known, and it is a known practice to use a phase change material in such systems for the purpose of storing the energy of the sun as disclosed, for example, in Ashote U.S. Pat. No. 4,131,158 and Lindner et al U.S. Pat. No. 4,371,029. It is also known that when a phase change material is utilized, heat conducting fins may be provided within the phase change material to facilitate the transfer of heat to and from such material as disclosed, for example, in the aforesaid Lindner et al patent and in Hepp U.S. Pat. No. 3,996,919. However, when such fins are used, it is conventional practice to dispose the fins in parallel planes extending radially from the conduit which conducts the heated liquid, and into the phase change material. While this fin arrangement serves the desired purpose of increasing the efficiency of the heat transfer to and from the phase change material, it significantly increases the manufacturing cost of the system because the relatively large number of thin, relatively closely spaced fins must be welded or otherwise individually fixed to the conduit at close spacings which require time-consuming and skilled labor. The system of the present invention overcomes the drawback of prior art arrangements by providing a fin arrangement that can be fabricated quickly and at a relatively small cost, and that has a unique interaction with a plurality of conduits for conducting heat transfer liquids therein.
In solar systems of the type generally described above, some attention has been given to a control system for increasing the efficiency of the system by shutting down the circulation of the fluid from the solar collector to the heat storage unit when the temperature in the solar panel liquid circuit in less than the temperature in the storage vessel. Thus, Saarem et al U.S. Pat. No. 4,191,166 discloses a closed system including a solar collector, a storage tank and a pump which is controlled to stop the circulation of the liquid flow from the collector to the storage tank when the temperature of the collector liquid is less than that in the storage tank, and also to stop such circulation if the storage tank temperature exceeds a predetermined limit. A somewhat similar control for a closed circulating system is disclosed in Buckley et al U.S. Pat. No. 4,399,807 where an automatic overtemperature control is provided to protect the collector in the event of the liquid circulation through the collector falls below a predetermined limit. Kirts U.S. Pat. No. 4,339,930 discloses a solar system having a collector, a domestic water heater, a heat exchanger, a heat pump, and a thermal storage unit, and a somewhat complicated control system that operates single pump for the entire system and a plurality of valves so that the fluid flow to and from the collector is routed through the several aforesaid components of the system as a result of the differentials and relationship of the sensed temperature in the water heater, the collector and thermal storage system. In contrast to these known control arrangements, the system of the present invention includes a combined heat exchange and storage unit, and separate fluid circuits for the collector and the water heater which are independently controlled to significantly improve the efficiency of the overall system.
Another problem frequently encountered in solar collector systems utilizing a phase change material such as paraffin is the problem of properly containing the phase change material while still providing an effective means for accommodating the substantial volumetric change of the phase change material when the phase change occurs (e.g., from solid to liquid, and from liquid to solid). One known solution to this problem has been to encase the phase change material within the confines of specially formed flexible tubular containers as disclosed in Farfaletti-Casali et al U.S. Pat. No. 4,362,207 or Wasserman U.S. Pat. No. 4,250,958, or to provide a biased compression member that permits expansion and contraction of the phase change material while maintaining it under compression as disclosed in Van Heel U.S. Pat. No. 4,270,523, or to load the phase change material into individual sealed pouches or bags formed of a resilient plaster material as disclosed in Stice U.S. Pat. No. 4,421,101 and Cordon U.S. Pat. No. 4,367,788. While all of these methods of containing the phase change material appear to protect the phase change material from undesirable and potentially destructive contact with ambient air while accommodating for the above-described volumetric variations in the phase change material, they are also relatively expensive in terms of initial cost and installation. In the system of the present invention, a simple and effective arrangement is provided which allows for significant volumetric variations in the phase change material while still permitting the material to be easily loaded into a rigid container which is free of contamination by ambient air.
Finally, solar collector systems often utilize certain types of heat transfer liquids other than water which are designed to increase the efficiency of the operation of the system, and some of these liquids, such as propyl glycol, will in time become acidic. When this happens, the acidic nature of the liquid will tend to cause deterioration and corrosion of the conduits, fittings, seals and other components exposed to the liquid. Heretofore, this problem has been dealt with by the costly and time consuming procedure of periodically flushing the acidic liquid from the system and replacing it with a fresh supply, and, in some cases, the replacement of the fluid is done only after the fluid has become acidic and damage to the system components has already occurred. The present invention provides a very simple and inexpensive addition which will automatically neutralize the acid in the heat transfer liquid shortly after it become acidic, without the necessity of flushing or replacing such liquid.